IN OUCHIDE/E AND ASCLEPIADEiE. 527 



opportunities of examining during the summer. For in 

 those species in which the pollen mass was not found 

 transferred from its original position to the fissure, and in 

 contact with the base of the style, no doubt by means of 

 insects, it was not difficult to place it there ; and in doing 

 so I never failed to obtain the same results. 



I now turned my attention to the base of the stigma, 

 expecting to find there such a modification of surface as 

 might serve to account for the rupture and production of 

 the tubes in the mass brought in contact witli it. I have, 

 however, in no case been able to observe the slightest [737 

 appearance of secretion, or any difl'erence whatever in 

 texture, between that part and the general surface of the 

 stigma. 



The bursting of the mass in Asclepias is uniformly on 

 the more rounded edge ; and this, it may be observed, is 

 the inner edge or margin of the mass, with reference to the 

 cell of the anthera in which it is formed ; and I may 

 further remark, that in the only case in which 1 have 

 hitherto observed dehiscence in an erect pollen mass, 

 namely, in Hoya carnosa, it also takes place along the 

 inner margin. 



In Asclepias the bursting always commences at the most 

 prominent point of the convex edge, and to this part it is 

 generally confined : it is sometimes, however, found ex- 

 tending through the greater part of its length. 



On carefully examining the convex edge, and more par- 

 ticularly its most prominent portion, I have not been able 

 to observe in it any change or peculiarity of texture, or 

 even any obvious difference in the form of the meshes of 

 the reticulated surface. Notwithstanding this apparent 

 want of secretion in the base of the stigma, and of differ- 

 ence of texture in the covering of the mass of pollen at the 

 point where it comes in contact with that organ, it must 

 still be supposed that there is some peculiarity both in the 

 surface of the stigma and in the prominent edge of the 

 mass, on which the effects in question depend. 



These effects are indeed very remarkable ; the stinmlus 

 here supposed to be derived from the surface of the stigma, 



